Bleaching polyacrylonitrile



8-111 SR P15 8309 XE 2;?321443 ratemeu U85. 0, Lil-II BLEACHINGPOLYACRYLONITRILE John Cadwallader Richards, Kenmore, N. Y., assignor toE. I. du Pont de Nemours & Company, Wilmington, Del., a corporation ofDelaware No Drawing. Application January 10, 1945, Serial No. 572,261

6 Claims.

This invention relates to the removal of color from shaped articlescomprising a polymer of acrylonitrile and is particularly concerned withthe bleaching of yarn composed of acrylonitrile polymer.

Polymers of acrylonitrile such as acrylonitrile polymers in which thepolymers are composed entirely of acrylonitrile, and interpolymers andcopolymers of acrylonitrile with other materials such as vinyl chloride,vinyl acetate, acrylic acid, its esters and homologues, styrene,butadiene, isobutylene and other polymerizable substances, especiallythose containing olefinic and diolefinic linkages, although potentiallyof great value on account of their highly desirable properties, areinsoluble in ordinary organic solvents when the polymers contain atleast 85% by Weight of acrylonitrile (vinyl cyanide) in the polymermolecule. Solutions of such polymers in organic solvents such asdimethyl formamide and other organic solvents, as well as theirconversion by wet and dry or evaporative processes into various articlessuch as filaments, films, etc. are described and claimed in thecopending applications of George Henry Latham, Serial No. 447,466, filedJune 17, 1942, and Ray Clyde Houtz, Serial No. 447,446, filed June 17,1942. This invention is broadly concerned with polymers of acrylonitrilecontaining at least 85% by weight of acrylonitrile in the polymermolecule although it is most particularly concerned with and will bespecifically described in connection with polyacrylonitrile, i. e. thepolymer which is composed substantially entirely of acrylonitrile in thepolymer molecule.

One method which may be used to produce polyacrylonitrile yarn is theso-called wet spinning process which comprises extruding a solution ofpolyacrylonitrile through a perforated nozzle (spinneret) into a bathcomposed of a liquid which will readily leach out the solvent from thepolymer solution and coagulate or precipitate the polymer from itssolution, the yarn being carried through the bath for a period of timesufiicient to solidify the polymer to the desired extent and beingsubjected, if desired, to a stretching operation, preferably while it isin the gel state, in order to increase tenacity as well as .otherwise toimprove physical properties by orienting the micelles of which thefilaments are composed. Yarn produced by means of this wet spinningprocess has excellent physical properties, but does not possess thatdegree of freedom from color which is required by the exacting standardsof the textile industry. Moreover, the

customary methods utilized in the textile art for.

the bleaching of color from filaments, fibers and the like do not efiectthe desired elimination of color from polyacrylonitrile yarn. Theyellowish or golden-brown color present in polyacrylonitrile articlesoccurs either during polymerization or processing of the polymer priorto or during shaping of the polymer to the desired form and may becaused by prolonged treatment of the polymer at high temperature, e. g.C. and above.

The invention will be described most particularly with reference to yarncomposed of continuous filaments, but it will b 'understood that this ismerely by way of illustration since the invention may also be applied tostaple fibers, thin transparent films such as may be used as wrappingtissue, monofils, artificial straw, hollow pellicles such as tubing, andother industrialproducts composed of acrylonitrile polymer.

It is an object of this invention to provide a process for. removingcolor .from articles composed of acrylonitrile polymer. It is aparticular object to produce color-free acrylonitrile polymer filaments,fibers and yarns without substantially impairing th physical propertiesof the products. Other objects will appear hereinafter.

The objects of the invention are accomplished in general by extruding asolution of polyacrylonitrile through a spinneret submerged in a liquidwhich will rapidly coagulate the polymer, passing the bundle offilaments through the bath of coagulating liquid for a distancesufficient to solidify or set the filaments, preferably collecting theyarn so formed on a bobbin or other suitable collecting device,maintaining it in the gel state, i. e. before the gel structure hascollapsed, as for example while the yarn still possesses a large amountof absorbed coagulating bath liquid, e. g. 100% or more by weight of theacrylonitrile polymer and soaking the gel yarn with an aqueous solutionof hydrogen peroxide or hypochlorous acid (which function to removecolor by oxidation) in the critical concentrations set forth hereinafterat a temperature of 50 to 100 C. for a short period of time.Objectionable coloring present inthe gel yarn is removed by thebleaching treatment with no substantial impairment of the desiredphysical properties of the yarn.

The following examples in which'parts, percentages and proportions areby weight unless otherwise specified and which are intended to beillustrative instead of limitative, set forth preferred forms of theinvention.-

EXAMPLE A Yarn to be treated in accordance with the present inventionmay be prepared as follows:

A solution of 18 parts of polyacrylonitrlle possessing an averagemolecular weight of 120,000, as determined from viscosity measurementsby the Staudinger formula, in 82 parts of dimethyl formamide is extrudedat a temperature of 100 0. through a ten-hole spinneret into a spinningbath consisting of glycerol heated to"'a temperature of C., the solutionbeing ex- 153 wa 1 i.

denier by means of a snubbing tension guide .5

comprising two stationary pins mounted at a distance of 16 inches fromthe spinneret face.

On leaving the bath, the yarn is passed without slippage about apositively driven feed :wheel possessing a peripheral speed of 3,600inches per minute, the yarn being subsequently collected'on a rotating,perforated bobbin possessing thesame peripheral speed as the feed wheel.The bobbin and gel yarn wound thereon are then placed on a chuck andsubmitted to pressure washing with water until the glycerol is removedfrom the yarn. Where skeined yarn is desired, the yarn is wound intoskeins from the collectingbobbin and washed in running water until freefrom glycerol. The yarns used in the following examples have been freedfrom glycerol in the manner described in this paragraph, the retainedwash water maintaining the yarn in the swollen condition.

Example I Acrylonitrile polymer yarn, prepared by a process such as thatdescribed'in Example A and Wound on the perforated collecting bobbinwhich permits the passage of fluid through the bobbin and yarn, .issoaked, while still in the gel state, in water containing hydrogenperoxide at a temperature of 75 C. for ten minutes, is washed withdistilled water and dried at 40 0.; the dried yarnis then'heat-treatedwhile relaxed and free to shrink at 105 C. ,forfive minutes, to increaseya nelongation. As compared with completely white yarn in which thecolor is designated as zero, the yarn bleached in accordance with thisexample shows a color of 0.5 as contrasted with dried yarn (otherwiseprepared and processed in thesame way except that the bleachingtreatment was omitted), exhibiting a color of .4, and the dry tenacityof 5.2 grams per denier and the dry elongation of about 18% possessed bythe dried, unbleached yarn are substantially unaf fected'by thebleaching treatment.

Example 11 Skeins of gel acrylonitrile polymer yarn prepared by aprocesssimilar to Example A are subjected "to soaking in a 10% aqueous solutionof hydrogen peroxide for five minutes at 80 (3., followed by washingwith distilled water, drying and heat-treating as in Example I, andshow-a color of'0.5 on the same color scale used in Example I. The samedegree of freedom from color isobtained by soaking the skeins ofyarn-for one minute at 80 C. in the same aqueous hydrogen peroxidesolution. The bleaching treatment effects no marked change in yarntenacity and elongation.

Example III Yarn skeins identical with the'gel, unbleached yarn used inExample 11 are soaked, while still in the gel state, for ten minutes at80 C. in 3% aqueous solution of hydrogen peroxide, are washedindistilled water, dried and heat-treated as in Example I, and show acolor of 0.5 on the same color scale used in Example I, with nosubstantial decrease in tenacity and elongation. To show the effect ofconcentration of hydrogen peroxideon the removal of color, the sameyarn, in the form of skeins, is subjected to 1% aqueous hydrogenperoxide solution for one minute, followed by washing and drying, andshows a a color in water.

4 of 1.0 -with.,little,if any, :change in tenacity and elongation.

Example IV Yarn skeins identical with the gel, unbleached.yarn-processedin accordance with Examples 11 and III are soaked, whilestill in the gel state,

fforgfive'minutes at a temperature of 75 C. in an aqueous-solutionprepared by dissolving sodium hypochlorite inwater in an amountsufficient to provide a 0.5% concentration of availablechlorine,concentratedhydrochloric acid being added 'in anamQuntsuflicient to bring the pH of the solution -to 2.7. The yarn, afterwashing with distilled water and drying, but with no heattreatmentsubsequent to drying, shows a color of 0.0 on the same scale used inExamples'I to III, with no 'marked adverse effect on tenacity andelongation.

' Example V 'Acrylonitrile polymer yarn similar to thatproducedinaccordance with Example A and wound on a perforated bobbin, is soaked,while still'in the gel state, on the bobbin, in 2.5 liters of thehypochlorite solution of Examples IVby heating the yarn and solution ina beaker for twenty minutes until a temperature of 68 C. was reached.This temperature is maintained :2 C. for ten minutes, after which theyarn :is thoroughly washed with tap water, twisted, skeined, dried andsubjected, while relaxed, to heat-treatment at a temperature'of 114 'C.for twelve .minutes. Yarn tenacity and elongation are not materiallyaffected by the bleaching treatment and the-bleached yarn is almostcompletely white ascompared with-the light goldenbrown color of the sameyarn which had not been subjected to the bleaching treatment.

Thebleaching treatments of Examples .I-to V are all carried out atatmospheric pressure.

The acrylonitrile polymer for use with this invention is preferablyprepared by the ammonium persulfate catalyzed polymerization ofmonomeric acrylonitrile dissolved or emulsified It can, however, .beprepared by any other suitable type of polymerization reaction, such as*for example the emulsion type reaction disclosed by U. S. Patent No.2,160,054 to Bauer et al. The polymer preferably possesses a molecularweight within the range of 15,000 to 250,000 'oreven higher, ascalculated from viscosity measurements :by the Staudinger equation:

. I 81 Molecular we ght Kmc wherein:

and

C'=concentration of the solution expressed as :the

U iii weight (as calculated by the above equation) of approximately60,000. Increasing or decreasing the amount of the catalyst, whilemaintaining the other conditions constant, decreases or increases themolecular weight of the polymer.

Although the examples illustrate the use of glycerol as the coagulatingmedium in the wet spinning process for forming polyacrylonitrile yarn,other coagulating liquids, for example water, organic solvents such asalcohol, ether, etc., or aqueous solutions of salts, alkalies or acidsmay be used. The tension spinning of acrylonitrile polymers fromsolution, involving the use of coagulating baths which may, for example,comprise glycerol is described and claimed in the copending applicationof William W. Watkins, Serial No. 496,476, filed July 23, 1943.

The heat treatment imposed on the dried yarn as described in Examples Ito III and V is used to increase elongation, but yarns which are notheat treated may, of course, be bleached in accordance with theinvention as illustrated in Example IV. Reference is made to thecopending application of Shirleigh Silverman, Serial No. 496,396, filedJuly 28, 1943, which describes and claims the heat treatment ofacrylonitrile polymers to increase elongation.

In order to effect the bleaching action characterized by the presentinvention, the acrylonitrile polymer yarn must be wet spun and must bemaintained in the gel state up to and through the bleaching treatment;the coagulating bath liquid, for example glycerol, may, if desired, bereplaced before the bleaching treatment by another swelling liquid, forexample water and other liquids which can be used as coagulating bathliquids for acrylonitrile polymers, but the yarn must not be allowed tolose its swollen condition prior to bleaching. The examples illustratethe removal of glycerolby washing with water, this procedurerepresenting an inexpensive and preferred expedient to remove from theyarn relatively non-volatile materials present in the coagulating bath.

The bleaching solutions used in accordance with the present inventionmust be maintained within certain concentrations. Thus, if an aqueoussolution of hydrogen peroxide is used as the bleaching agent, thehydrogen peroxide must be within the range of 1% to based on the weightof the total aqueous solution and is preferably present in aconcentration of 3% to-10%. If aqueous solutions of hypochlorous acidare used, they must contain 0.05% up to 0.5% of available chlorine, withthe preferred concentration being in the neighborhood of 0.25%;hypochlorous acid solution may readily be prepared by dissolving amaterial such as sodium hypochlorite, or calcium hypochlorite, or othersoluble hypochlorite, in water, acidifying with a mineral acid such ashydrochloric or sulfuric acid to a pH of less than 7.0 and preferably5.0 or less, the term available chlorine signifying the chlorine presentas hypochlorous acid. An excess of mineral acid added to thehypochlorite solution does no harm and the pH can be as low as 2.0 oreven close to 0.0, if desired.

Adequate bleaching can be obtained in accordance with the presentinvention only by maintaining a temperature during the bleaching periodof between 50 and 100 C. The higher the temperature, the shorter thenecessary period of contact between the yarn and the bleaching solution.Normally, adequate bleaching is accomplished in a few minutes at thehigher temperature and in less than an hour at the lower end of thetemperature range. The preferred temperature is about 70 to C. and thepreferred time of bleaching is about one to thirty minutes.

Hydrogen peroxide and sodium hypochlorite have been used in very diluteconcentrations for the treatment of textile fibers such as cotton andrayon, but they have no useful effect, in the customary concentrationsandat temperatures below those used in the practice of thepresentinvention, as far as acrylonitrile polymer yarn is concerned.Alternatively, the use of the relatively concentrated solutions ofbleaching agent used in accordance with this invention would so de-vgrade the said textile materials as to make them practically useless. V

The wet spun, gel yarn of polyacrylonitrile may be bleached in asubstantially continuous manner after the coagulation thereof and beforethe yarn is collected on a bobbin, bucket or other collecting device.Alternatively, the gel yarn may be collected, for example on a bobbin ina bobbin spinning process, or in a bucket in a bucket spinning process,and the resultant gel yarn cake subjected to treatment with thebleaching solution. A preferred method of bleaching yarn in accordancewith this invention comprises skeining the yarn with the skeining reeldipping in water or even submerged under water to insure the retentionof the yarn in its swollen, gel state. The skein's of gel yarn are thendipped into the bleaching solution,

By the process of the present invention polyacrylonitrile yarn,filaments, fibers, film, monofilaments, artificial straw, hollow tubingand other structures can .be bleached to a satisfactory colorless orwhite character Without deleterious action on the structure. Thebleached polyacrylonitrile structures of the present invention, exceptfor improved color, exhibit substantially the same physicalcharacteristics as the unbleached structure.

Since it is obvious that many changes and modifications can be made inthe above described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to the details described herein except as set forth in theappended claims.

' I claim:

1. In the process of removing color from an article composed of apolymer of acrylonitrile containing at least by weight of acrylonitrilein the polymer molecule, the step which comprises subjecting saidarticle in the gel state, at a temperature of 50 to C. for a timesufiicient for bleaching and less than one hour to treatment with anaqueous solution taken from the class consisting of hydrogen peroxide in1% to 10% concentration by weight and hypochlorous acid in a,concentration corresponding to 0.05% to 0.5% by weight availablechlorine.

2. In the process of removing color from yarn composed of a polymer ofacrylonitrile containing at least 85% by weight of acrylonitrile in thepolymer molecule, the step which comprises subjecting Said yarn in thegel state, at a temperature of 50 to 100 C. and for a period of aboutone to thirty minutes, to treatment with an aqueous solution taken fromthe class consisting of hydrogen peroxide in 1% to 10% concentration byweight and hypochlorous. acid in a concentration corresponding to 0.05%to 0.5% by weight available chlorine,

3. In the process of removing color from yarn composedv ofpolyacrylonitrile, the step which comprises subjecting said yarn in thegel state, at a temperature of 50 to 100 C. for a time sufiicient forbleaching and less than one hour, to treatment with an aqueous solutionof hydrogen peroxide in 1% to 10% concentration by weight.

4. In the process of removing color from yarn composed ofpolyacrylonitrile, the step which comprises subjecting said yarn in thegel state, at a; temperature of 50 to 100 C. for a time sufiicient forbleaching and less than one hour, to treatment with an aqueous solutionof hypochlorous acid in a concentration corresponding to 0.05% to 0.5%by Weight available chlorine.

5. In the process of removing color from yarn composed ofpolyacrylonitrile, the step which comprises subjecting said yarn in thegel state, at a temperature-of 70 to 80 C. for a time sufficient forbleaching and less than one hourfto treatment with an aqueous solutionof hydrogen peroxide in 3% to 10% concentration by weight.

6. In the process of removing color from yarn composed ofpolyacrylonitrile, the step which 8 comprises subjecting said yarn inthe gel state, at atemperature of 70 to 80 C. for a time sufiicient forbleaching and less than one hour, to treatment with an aqueous solutionof hypochlorous

